network default

If your wireless network has a dhcp-server and you get your parameters dynamicly when connecting, just add this line into your /etc/rc.conf file:
ifconfig_ath0="DHCP"

And if you specify parameters by yourself, add this lines:
defaultrouter=""
ifconfig_ath0="ssid inet netmask "

and to your /etc/resolv.conf:
nameserver

And that's it, you would not need to do anything manually anymore.